1. Field of the Invention
This invention relates to a process for preparation of electrophotographic pigments and the use of such pigments in electrophotographic imaging elements and methods. More specifically, this invention provides a novel route for the preparation of the X polymorph of metal-free phthalocyanine from the alpha form of this pigment.
2. Description of the Prior Art
The formation and development of images on the imaging surface of photoconductive materials by electrostatic means is well-known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on an imaging surface of an imaging member by first uniformly electrostatically charging the surface of the imaging member in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light struck areas of the imaging layer are thus rendered conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image bearing surface is rendered visible by development with a finely divided colored electroscopic powder material, known in the art as "toner". This toner will be principally attracted to those areas on the image bearing surface which retain the electrostatic charge and thus form a visible powder image.
The developed image can then be read or permanently affixed to the photoconductor in the event that the imaging layer is not to be reused. This latter practice is usually followed with respect to the binder-type photoconductive films where the layer is an integral part of the finished copy.
In so-called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to another surface, such as a sheet of paper, and thereafter developed. When the latent image is developed on the imaging surface of a reusable photoconductor, it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well-known techniques can be used to permanently affix the toner image to the copy sheet, including overcoating with transparent films, and solvent or thermal fusion of the toner particles to the supportive substrate.
In the above "plain paper" copying systems, the materials used in the photoconductive layer should preferably be capable of rapid switching from insulative to conductive to insulative state in order to permit cyclic use of the imaging layer. The failure of the photoconductive material to return to its relative insulative state prior to the succeeding charging sequence will result in an increase in the rate of dark decay of the photoconductor. This phenomenon, commonly referred to in the art as "fatigue", has in the past been avoided by the selection of photoconductive materials possessing rapid switching capacity. Typical of the materials suitable for use in such a rapidly cycling imaging system include anthracene, sulfur, selenium and mixtures thereof (U.S. Pat. No. 2,297,691); selenium being preferred because of its superior photosensitivity.
In addition to anthracene, other organic compounds, such as phthalocyanine pigments, are also reportedly useful in electrophotography, see for example U.S. Pat. No. 3,594,163. These pigments can generally be classified into two major subgroups; the metal-free phthalocyanines and the metal-containing phthalocyanines. X-ray diffraction studies and/or infrared spectral analysis of these pigments indicate that phthalocyanines also exist in at least two different polymorphic forms; they being designated alpha and beta -- (listed in order of increasing stability). In addition to these well-known forms of the metal-free and metal-containing phthalocyanines, additional polymorphs of the metal-containing phthalocyanines have also been recently reported, U.S. Patents 3,051,721 (R-form); 3,160,635 (delta-form); and 3,150,150 (delta-form).
More recently, an additional polymorph of the metal-free and metal-containing phthalocyanine pigments has been disclosed. This polymorph, being designated the X form, is described and methods for its preparation contained in U.S. Pats. Re. Nos. 27,117; 3,657,272; and 3,594,163. Comparative evaluation of the various forms of phthalocyanine pigments for use in electrophotography has revealed the X form to be preferred because of its superior electrophotographic speed. The potential use of this polymorphic form of phthalocyanine pigments in electrophotographic systems imposes stringent requirements on the purity of this material. It is, therefore, imperative that the techniques employed in synthesis of this form of pigment insure that the resulting product be free of impurities and/or other contaminates which can interfere with the electronic requirements of an electrophotographic imaging system.
Until recently, phthalocyanine has been prepared almost exclusively for use as a pigment, where color, tinctorial strength, light fastness, dispersability, etc. are prime considerations and the purity of the pigment being of only incidential importance. As a result of this emphasis, the reported methods for synthesis of these compounds very often introduce metals and/or other complex organic materials into the pigment which are very difficult to remove; see Moser and Thomas, Phthalocyanine Compounds, Reinhold Publishing Co., p.p. 104 - 189. Two of the more common methods used in the manufacture of phthalocyanine pigments generally involve (1) indirect formation of the pigment for an acid and a metal phthalocyanine containing a replaceable metal and (2) direct synthesis from phthalonitrile.
Accordingly, it is, the object of this invention to provide a process for preparation of the X form of metal-free phthalocyanine substantially free of the contaminants and impurities associated with its preparation by more conventional prior art techniques.
More specifically, it is the principal object of this invention to provide a process for the preparation of the X form of metal-free phthalocyanine from alpha metal-free phthalocyanine.
It is another of the objects of this invention to provide a process which is directive for the synthesis of the X form of metal-free phthalocyanine.
It is yet another object of this invention to provide a process which is directive for the preparation of the X form of metal-free phthalocyanine from the alpha form of metal-free phthalocyanine.
It is yet a further object of this invention to provide a process for the preparation of the X form of metal-free phthalocyanine in thin compact films.